The expression of activated cellular oncogenes in chemically induced rat tumors and the relationship of oncogene expression to progression from the normal to the neoplastic phenotype are studied using 3T3 transfection and hybridization techniques and monoclonal antibodies directed against the specific oncogene products. Five types of tumors have been generated by single injection of F344 rats using various alkylating agents: renal mesenchymal tumors induced by methyl (methoxymethyl) nitrosamine (DMN-OMe), intestinal adenocarcinomas induced by methyl-(acetoxymethyl) nitrosamine (DMN-OAc), hepatocellular carcinomas induced by intraportal injection of DMN-OAc followed by phenobarbital promotion, and gliomas and schwannomas induced by transplacental exposure to nitrosoethylurea (ENU). DNA purified from these tumors is utilized for 3T3 transfection assays and in Southern blot hybridizations with available oncogene probes. Selective activation of neu, proved to result from a single base T leads to A transversion mutation at one specific site, was observed in 3T3 transformants and in DNA from primary tumors and was shown in 11 of 12 schwannomas tested, but in no other kinds of tumors. K-ras was selectively activated in renal mesenchymal tumors, but no specific and consistent association with a specific activated oncogene was seen in central nervous system gliomas, intestinal adenomas and carcinomas, or hepatocellular tumors. Using monoclonal and polyclonal antibodies, H-ras p21 was found in normal and neoplastic tissues dependent on the fixative and antisera used. Patterns of specific and nonspecific staining were characterized and applications of these antisera were developed. With one monoclonal antibody, granules (probably mitochondria) were immunostained in many normal tissues including renal tubules, muscle and brain.